Retired astronaut Mark Kelly says he's running to finish John McCain's term in the U.S. Senate.

Kelly announced his plans Tuesday in a video message he tweeted. He's married to former Democratic Congresswoman Gabrielle Giffords, who survived being shot in the head during a constituent event in Tucson eight years ago.

Kelly is a top Democratic recruit to challenge Republican Sen. Martha McSally, who was appointed to the seat late last year. The race is widely expected to be one of the most closely contested Senate races of the 2020 election.

Kelly has never held elected office but became a visible advocate for gun control after his wife was shot.